Duplexer and front-end circuit

ABSTRACT

A duplexer includes first and second filter circuits and first and second wirings. The first filter circuit allows a signal of a first frequency band to pass therethrough between a first terminal and a common terminal and includes a first resonator which is connected at one end to a line disposed between the first terminal and the common terminal to branch off from the line. The second filter circuit allows a signal of a second frequency band, which is different from the first frequency band, to pass therethrough between a second terminal and the common terminal. The first wiring is connected at one end to the common terminal and is opened at the other end. The second wiring is connected at one end to the other end of the first resonator and is grounded at the other end. The first wiring is electromagnetically coupled with second wiring.

This application claims priority from Japanese Patent Application No.2018-091512 filed on May 10, 2018; Japanese Patent Application No.2017-200748 filed on Oct. 17, 2017; and Japanese Patent Application No.2017-148334 filed on Jul. 31, 2017. The content of these applicationsare incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a duplexer and a front-end circuit. Incommunication devices, such as cellular phones, a duplexer whichseparates a transmit signal and a received signal from each other isused when signals are transmitted and received via one antenna.International Publication No. 2007/102560 discloses the followingduplexer, for example. In this duplexer, a transmit filter that allows asignal of a transmit frequency band to pass therethrough and a receivefilter that allows a signal of a receive frequency band to passtherethrough are each constituted by a ladder filter. In such aduplexer, a leakage of a signal from the transmit filter to the receivefilter may occur. It is thus desirable to improve the isolationcharacteristics between the transmit filter and the receive filter. Inthis duplexer, a first wiring connected between an antenna terminal anda ground and a second wiring connected between a resonator included inthe ladder filter and a ground are electromagnetically coupled with eachother so as to improve the isolation characteristics.

BRIEF SUMMARY

In the above-described duplexer, however, it is necessary to arrange thefirst and second wirings, which are parallel with each other, so thatcurrents may flow in the first and second wirings in oppositedirections. This decreases the flexibility in the arrangement of thefirst and second wirings.

In view of the above-described background, the present disclosureprovides a duplexer and a front-end circuit that are capable ofimproving isolation characteristics between plural filters whileincreasing the flexibility in the arrangement of wirings.

According to one aspect of the present disclosure, there is provided aduplexer including first and second filter circuits and first and secondwirings. The first filter circuit allows a signal of a first frequencyband to pass therethrough between a first terminal and a commonterminal. The first filter circuit includes a first resonator which isconnected at one end to a line disposed between the first terminal andthe common terminal so as to branch off from the line. The second filtercircuit allows a signal of a second frequency band to pass therethroughbetween a second terminal and the common terminal. The second frequencyband is different from the first frequency band. The first wiring isconnected at one end to the common terminal and is opened at the otherend. The second wiring is connected at one end to the other end of thefirst resonator and is grounded at the other end. The first and secondwirings are electromagnetically coupled with each other.

According to another aspect of the present disclosure, there is provideda duplexer including first and second filter circuits and first andsecond wirings. The first filter circuit allows a signal of a firstfrequency band to pass therethrough from a first terminal to a commonterminal. The first filter circuit includes a first resonator which isconnected at one end to a line disposed between the first terminal andthe common terminal so as to branch off from the line. The second filtercircuit allows a signal of a second frequency band to pass therethroughfrom the common terminal to a second terminal. The second frequency bandis different from the first frequency band. The first wiring isconnected at one end to the common terminal and is opened at the otherend. The second wiring is connected at one end to the other end of thefirst resonator and is grounded at the other end. The first and secondwirings are electromagnetically coupled with each other.

According to still another aspect of the present disclosure, there isprovided a front-end circuit including first through third filtercircuits, a switch circuit, and first and second wirings. The firstfilter circuit allows a first transmit signal to pass therethroughbetween a first terminal and a common terminal. The second filtercircuit allows a first received signal to pass therethrough between asecond terminal and the common terminal. The third filter circuit allowsa second transmit signal and a second received signal to passtherethrough between third and fourth terminals. The third filtercircuit includes a first resonator which is connected at one end to aline disposed between the third and fourth terminals so as to branch offfrom the line. The switch circuit connects one of or both of the commonterminal and the fourth terminal to an antenna terminal. The firstwiring is connected at one end to the fourth terminal and is opened atthe other end. The second wiring is connected at one end to the otherend of the first resonator and is grounded at the other end. The firstand second wirings are electromagnetically coupled with each other.

According to embodiments of the disclosure, it is possible to provide aduplexer and a front-end circuit that are capable of improving isolationcharacteristics between plural filters while increasing the flexibilityin the arrangement of wirings.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of embodiments of the present disclosure with reference tothe attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating the configuration of aduplexer according to a first embodiment of the disclosure;

FIG. 2 is a circuit diagram illustrating the configuration of a filtercircuit and that of stubs in the duplexer of the first embodiment;

FIG. 3 is a plan view of a main surface of a multilayer substrate onwhich the duplexer of the first embodiment is formed;

FIG. 4 is a sectional view taken along line IV-IV of FIG. 3;

FIG. 5 is a circuit diagram illustrating the configuration of a filtercircuit and that of stubs in a duplexer according to a second embodimentof the disclosure;

FIG. 6 is a plan view of a main surface of a multilayer substrate onwhich the duplexer of the second embodiment is formed;

FIG. 7 is a graph illustrating the simulation results of isolationcharacteristics between filter circuits in the duplexer of the firstembodiment;

FIGS. 8A and 8B are plan views illustrating modified examples regardingthe arrangement of an open stub and a short stub;

FIG. 9 is a graph illustrating the simulation results of isolationcharacteristics between filter circuits in the configurations shown inFIGS. 8A and 8B;

FIG. 10 is a conceptual diagram illustrating the configuration of aduplexer according to a third embodiment of the disclosure;

FIG. 11 is a Smith chart illustrating a path of output impedance of afilter circuit in the duplexer of the third embodiment;

FIG. 12 is a conceptual diagram illustrating the configuration of aduplexer according to a fourth embodiment of the disclosure;

FIGS. 13A through 13C are sectional views illustrating modified examplesregarding the arrangement of an open stub and a short stub;

FIG. 14 is a plan view illustrating a modified example regarding thearrangement of an open stub and a short stub;

FIGS. 15A and 15B are plan views illustrating modified examplesregarding the arrangement of an open stub and a short stub;

FIG. 16 is a graph illustrating the simulation results of isolationcharacteristics between filter circuits in the configurations shown inFIGS. 15A and 15B;

FIGS. 17A and 17B are plan views illustrating modified examplesregarding the line-width of a short stub;

FIG. 18 is a conceptual diagram illustrating the configuration of aduplexer according to a fifth embodiment of the disclosure;

FIG. 19 is a circuit diagram illustrating the configuration of a filtercircuit and that of stubs in the duplexer of the fifth embodiment;

FIG. 20 is a circuit diagram illustrating the configuration of a filtercircuit and that of stubs in a duplexer according to a sixth embodiment;

FIG. 21 is a plan view of a main surface of a multilayer substrate onwhich the duplexer of the sixth embodiment is formed;

FIG. 22 is a circuit diagram illustrating the configuration of a filtercircuit and that of stubs in a duplexer according to a seventhembodiment;

FIG. 23 is a plan view of a main surface of a multilayer substrate onwhich the duplexer of the seventh embodiment is formed;

FIG. 24 is a graph illustrating the simulation results of isolationcharacteristics between filter circuits in the duplexer of the fifthembodiment;

FIG. 25 is a graph illustrating the simulation results of isolationcharacteristics between filter circuits in the duplexer of the seventhembodiment;

FIG. 26 is a conceptual diagram illustrating the configuration of aduplexer according to a modified example of the fifth embodiment;

FIG. 27 is a conceptual diagram illustrating the configuration of aduplexer according to another modified example of the fifth embodiment;and

FIG. 28 is a conceptual diagram illustrating the configuration of afront-end circuit according to an eighth embodiment of the disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below in detailwith reference to the accompanying drawings. The same element isdesignated by like reference numeral, and an explanation thereof will begiven only once and will not be repeated.

FIG. 1 is a conceptual diagram illustrating the configuration of aduplexer 100A according to a first embodiment of the disclosure. Theduplexer 100A is used in a communication device, such as a cellularphone. In transmitting and receiving radio frequency (RF) signals of apredetermined frequency band via one antenna, the duplexer 100A servesthe function of separating a transmit signal and a received signal fromeach other. More specifically, the duplexer 100A includes two filtercircuits 10 and 11, an antenna terminal 20, a common terminal 21, atransmit terminal 22, a receive terminal 23, and two stubs 30 and 31.

A transmit signal is supplied from a transmit circuit (not shown) to thefilter circuit 10 (first filter circuit) via the transmit terminal 22(first terminal). The filter circuit 10 allows a signal of apredetermined frequency band (first frequency band) to pass therethroughbetween the transmit terminal 22 and the common terminal 21 andattenuates signals of the other frequency bands. The signal passingthrough the filter circuit 10 further passes through the common terminal21 and the antenna terminal 20 and is transmitted from an antenna (notshown) connected to the antenna terminal 20. Details of theconfiguration of the filter circuit 10 will be discussed later.

A received signal received by the antenna is supplied to the filtercircuit 11 (second filter circuit) via the antenna terminal 20 and thecommon terminal 21. The filter circuit 11 allows a signal of apredetermined frequency band (second frequency band) to passtherethrough between the common terminal 21 and the receive terminal 23(second terminal) and attenuates signals of the other frequency bands.The received signal passing through the filter circuit 11 is supplied toa receive circuit (not shown), for example.

The pass band of the filter circuit 10 and that of the filter circuit 11are not restricted to particular bands, but they are different from eachother. If the duplexer 100A is used for transmitting and receiving RFsignals of Band 8, for example, the pass band of the filter circuit 10is about 880 to 915 MHz, which is the transmit band of Band 8, while thepass band of the filter circuit 11 is about 925 to 960 MHz, which is thereceive band of Band 8. That is, the pass band of the filter circuit 11is higher than that of the filter circuit 10. However, the pass band ofthe filter circuit 10 may be higher than that of the filter circuit 11.The filter circuits 10 and 11 may be formed on different chips or may beformed on the same chip.

The stub 30 (first wiring) is an open stub which is connected at one endto the common terminal 21 and is opened at the other end. In thefollowing description, the stub 30 will also be called the open stub 30.

The stub 31 (second wiring) is a short stub which is connected at oneend to a resonator included in the filter circuit 10, which will bediscussed later, and is grounded at the other end. In the followingdescription, the stub 31 will also be called the short stub 31. Althoughthe short stub 31 is depicted outside the filter circuit 10 for the sakeof representation, it may be one of the elements included in the filtercircuit 10.

Each of the open stub 30 and the short stub 31 may be constituted by alumped element, such as an inductance element or a resistance element.Each of the open stub 30 and the short stub 31 may alternatively beconstituted by a distributed element, such as wiring having inductancecomponents. The open stub 30 and the short stub 31 are at leastpartially magnetically coupled with each other (magnetic-field coupling)by electromagnetic induction. In the specification, “coupling” or “beingcoupled” is used in the following manner. In the first embodiment, forexample, when the isolation characteristics between the filter circuits10 and 11 are improved because of the provision of the open stub 30 andthe short stub 31, compared with the configuration without them, theopen stub 30 and the short stub 31 are coupled with each other. Couplingis not restricted to magnetic-field coupling and may be electric-fieldcoupling. The configuration of the filter circuit 10 and that of thestubs 30 and 31 will be discussed in greater detail with reference toFIG. 2.

FIG. 2 is a circuit diagram illustrating the configuration of a filtercircuit 10A and that of the stubs 30 and 31 in the duplexer 100A of thefirst embodiment. Although the open stub 30 and the short stub 31 arerepresented by the symbol of inductance elements in FIG. 2 for the sakeof representation, they are constituted by wirings. The stubsrepresented by the symbol of inductance elements in FIGS. 5, 19, 20, and22 are also constituted by wirings.

The filter circuit 10A is a ladder filter constituted by plural filterelements connected in series with each other and those connected inparallel with each other. More specifically, the filter circuit 10Aincludes four resonators 41 through 44 and three resonators 45 through47. The four resonators 41 through 44 are connected in series with eachother on a line L1 connecting the transmit terminal 22 and the commonterminal 21. The three resonators 45 through 47 are connected inparallel with each other between the line L1 and a ground. Although theconfiguration of the resonators 41 through 47 is not restricted to aparticular configuration, they are surface acoustic wave (SAW) filters,for example. The resonators 41 through 47 may alternatively be filters,such as piezoelectric thin-film resonators, bulk acoustic wave (BAW)filters, or incredible high performance (I.H.P.) SAW filters. The fourresonators 41 through 44 and the three resonators 45 through 47 areshown in FIG. 2. However, more or fewer series-connected resonators andmore or fewer parallel-connected resonators may be provided.

Among the parallel-connected resonators 45 through 47, the resonator 45(first resonator) positioned closest to the transmit terminal 22 isconnected at one end to the line L1 so as to branch off from the lineL1, and at the other end (terminal 24) to one end of the short stub 31.The short stub 31 is connected at one end to the terminal 24 and isgrounded at the other end. The resonator to which the short stub 31 isconnected is not limited to the resonator 45 and may be another one ofthe parallel-connected resonators 46 and 47. In the first embodiment,the open stub 30 and the short stub 31 are formed in or on a modulesubstrate 50 (multilayer substrate, for example). On the modulesubstrate 50, a chip on which the filter circuit 10A is formed ismounted.

The boundary of the block representing the filter circuit 10A and thatof the block representing the module substrate 50 in FIG. 2 are only anexample. For example, the node between the resonators 46 and 47 isincluded in the block representing the filter circuit 10A in FIG. 2.However, the resonators 46 and 47 may be connected to each other on achip on which the filter circuit 10A is formed or may be connected onthe module substrate 50. The boundary of the block representing a filtercircuit 10B and that of the block representing the module substrate 50shown in FIG. 5 are also only an example. Arranging of the open stub 30and the short stub 31 so as to be magnetically coupled with each otherwill be discussed below with reference to FIGS. 3 and 4.

FIG. 3 is a plan view of a main surface of a multilayer substrate 50A onwhich the duplexer 100A of the first embodiment is formed. FIG. 4 is asectional view taken along line IV-IV of FIG. 3. In FIG. 3, an open stub30 a and a short stub 31 a formed on different layers of the multilayersubstrate 50A overlap each other as viewed from above. For the sake ofrepresentation, the open stub 30 a is indicated by the hatched portionso that it can be distinguished from the short stub 31 a. The open stubis also indicated by the hatched portion in FIGS. 6, 8A, 8B, 14, 15A,15B, 21, and 23.

The multilayer substrate 50A shown in FIGS. 3 and 4 has a substantiallyrectangular main surface parallel with a plane defined by the X and Yaxes and a thickness parallel with the Z axis. On the main surface ofthe multilayer substrate 50A, plural terminals including the transmitterminal 22, the common terminal 21, and the terminal 24 shown in FIG.2, the filter circuits 10 and 11 (not shown), and the stubs 30 a and 31a are formed. The short stub 31 a is connected at one end to theterminal 24 and extends from the terminal 24 so as to turncounterclockwise in a spiral shape in a plan view of the main surface ofthe multilayer substrate 50A. The other end of the short stub 31 a isconnected to a ground via a via-hole. The open stub 30 a is connected atone end to the common terminal 21 and extends from the common terminal21 to the terminal 24 and further extends from the terminal 24 so as toturn counterclockwise in a spiral shape in accordance with thearrangement of the short stub 31 a. The other end of the open stub 30 ais opened.

As shown in FIG. 4, the open stub 30 a and the short stub 31 a areformed on different layers of the multilayer substrate 50A. Themultilayer substrate 50A is constituted by inner layers 51 and 52 and afront layer 53 sequentially stacked on each other. The open stub 30 a isformed in the front layer 53, while the short stub 31 a is formed in theinner layer 52. The open stub 30 a and the short stub 31 a are disposedso as to overlap each other in the Z-axis direction, in a plan view ofthe main surface of the multilayer substrate 50A. With thisconfiguration, the open stub 30 a and the short stub 31 a aremagnetically coupled with each other with the inner layer 52 interposedtherebetween. The multilayer substrate 50A may be constituted by morethan or fewer than three layers.

Referring back to FIG. 1, in the duplexer 100A, although the most partof a transmit signal usually flows from the common terminal 21 to theantenna terminal 20 (see the solid-line arrow in FIG. 1), some part ofthe transmit signal leaks to the filter circuit 11 on the receive sidevia the common terminal 21 (see the broken-line arrow in FIG. 1). Thetransmit signal also leaks to the filter circuit 11 due to theoccurrence of coupling in the signal path. If the frequency of thetransmit signal leaking to the filter circuit 11 is included in the passband of the filter circuit 11, the transmit signal passes through thefilter circuit 11 and is supplied to a low-noise amplifier (LNA)connected to the receive terminal 23. This may decrease the receiversensitivity. To address this issue, it is desirable that a duplexer havehigh isolation characteristics between filter circuits.

The duplexer 100A includes the short stub 31 as described above. In thefilter circuit 10A, capacitance components of the resonator 45 andself-inductance components of the short stub 31 form a series resonancecircuit. The duplexer 100A also includes the open stub 30 magneticallycoupled with the short stub 31, as described above. Mutual inductancecomponents are also generated in the short stub 31. With thisconfiguration, a signal having the resonant frequency of the seriesresonance circuit is short-circuited to a ground via this seriesresonance circuit. In the duplexer 100A, the attenuation of signals thatare not included in the pass band of the filter circuit 10A isincreased, compared with the configuration without the stubs 30 and 31.More specifically, adjusting of the self-inductance values of the stubs30 and 31 can attenuate signals of frequencies higher than the pass bandof the filter circuit 10A and also included in the pass band of thefilter circuit 11. The duplexer 100A is thus able to improve theisolation characteristics between the filter circuits 10 and 11,compared with the configuration without the stubs 30 and 31.

In the duplexer 100A, the direction of a current flowing through theopen stub 30 and that through the short stub 31 may be the samedirection or may be the opposite directions. This can increase theflexibility in arranging the stubs 30 and 31 to be formed in or on themultilayer substrate 50A and thus enhances the flexibility in designingthe duplexer 100A, compared with the configuration in which a short stubis connected to a common terminal, as disclosed in InternationalPublication No. 2007/102560. This configuration will be called theconfiguration of the related art.

In the duplexer 100A, instead of the short stub 31, the open stub 30 isconnected to the common terminal 21, and thus, it is not necessary toconnect one end of the stub connected to the common terminal 21 to aground. This also increases the flexibility in the arrangement of thestubs 30 and 31, compared with the configuration of the related art.

The impedance characteristics are less likely to be changed by theinsertion of an open stub than by that of a short stub. Hence, the openstub 30 can safely be provided in the duplexer 100A substantiallywithout necessarily increasing a change in the impedancecharacteristics.

By adjusting an area by which the open stub 30 and the short stub 31overlap each other in the Z-axis direction (such an area may hereinafterbe simply called the overlapping amount), the strength of couplingbetween the open stub 30 and the short stub 31 can be controlled. Forexample, changing of the line-length or the line-width of one of or bothof the stubs 30 and 31 can adjust the overlapping amount between thestubs 30 and 31.

If the pass band of the filter circuit 11 is lower than that of thefilter circuit 10, the self-inductance values of the stubs 30 and 31 maybe adjusted so that signals of frequencies lower than the pass band ofthe filter circuit 10 and also included in the pass band of the filtercircuit 11 can be attenuated.

The terminals used in the first embodiment are physical terminals forelectrically connecting the chip and the module substrate. However,“terminals” in the specification are not limited to physical terminals.“Terminals” include nodes representing electrical connection betweenelements indicated in a circuit diagram. It is now assumed, for example,that an antenna (not shown) and the filter circuits 10 and 11 are formedon the same chip and wiring from the antenna to the filter circuits 10and 11 is integrally formed. In this case, if the open stub 30 isconnected to any region of this wiring, the open stub 30 is assumed tobe connected to the common terminal 21 shown in FIG. 1.

FIG. 5 is a circuit diagram illustrating the configuration of a filtercircuit 10B and that of stubs in a duplexer 100B according to a secondembodiment of the disclosure. The same elements as those of the firstembodiment are designated by like reference numerals, and an explanationthereof will thus be omitted. In the second through eighth embodiments,only different points from the first embodiment will be described whileomitting a description of the same points as those of the firstembodiment, and similar advantages obtained by similar configurationswill not be discussed in the individual embodiments.

The duplexer 100B shown in FIG. 5 is different from the duplexer 100Ashown in FIG. 2 in that it also includes a stub 32.

The stub 32 (third wiring) is a short stub connected at one end to theother end (terminal 25) of the resonator 46 (second resonator) and thatof the resonator 47 and is grounded at the other end. In the followingdescription, the stub 32 will also be called the short stub 32. Theshort stub 32, as well as the short stub 31, is magnetically coupledwith the open stub 30.

FIG. 6 is a plan view of a main surface of a multilayer substrate 50B onwhich the duplexer 100B of the second embodiment is formed.

In the multilayer substrate 50B in FIG. 6, a short stub 32 a isconnected at one end to the terminal 25 and extends from the terminal 25so as to turn clockwise in a spiral shape in a plan view of the mainsurface of the multilayer substrate 50B. The other end of the short stub32 a is connected to a ground via a via-hole. The short stub 32 a, aswell as the short stub 31 a, is formed on a different layer from that ofan open stub 30 b. The open stub 30 b is connected at one end to thecommon terminal 21 and extends to overlap at least part of the shortstub 31 a and at least part of the short stub 32 a in the Z-axisdirection. With this configuration, the open stub 30 b is magneticallycoupled with each of the short stubs 31 a and 32 a.

The duplexer 100B configured as described above also achieves advantagessimilar to those of the duplexer 100A. The duplexer 100B also includesthe short stub 32 a so that it can further attenuate signals that arenot included in the pass band of the filter circuit 10B. The duplexer100B can thus further improve the isolation characteristics than theduplexer 100A.

FIG. 7 is a graph illustrating the simulation results of isolationcharacteristics between the filter circuits in the duplexer 100A. Morespecifically, FIG. 7 shows the results of comparing the isolationcharacteristics between the filter circuits 10 and 11 in theconfiguration of the duplexer 100A with the open stub 30 and those ofthe configuration without the open stub 30. In this graph, the verticalaxis indicates the isolation characteristics (dB), while the horizontalaxis indicates the frequency (MHz).

FIG. 7 shows that, in the configuration with the open stub 30, theisolation characteristics are significantly improved at and around 950MHz within the pass band (925 to 960 MHz) of the filter circuit 11, andthe isolation characteristics of about −60 dB or more (the absolutevalue of decibel is 60 or higher) is implemented in the overall passband. At 960 MHz, for example, the isolation characteristics areimproved by about 10 dB, compared with the configuration without theopen stub 30. In the transmit frequency band (880 to 915 MHz), almost nodifferences are observed in the isolation characteristics between theconfiguration with the open stub 30 and that without the open stub 30.That is, the isolation characteristics in the receive frequency band canbe improved by the insertion of the open stub 30 substantially withoutnecessarily influencing the transmit frequency band.

FIGS. 8A and 8B are plan views illustrating modified examples regardingthe arrangement of the open stub 30 and the short stub 31.

On a multilayer substrate 50C shown in FIG. 8A, an open stub 30 c isdisposed so that the extending direction of the open stub 30 c from atone end connected to the common terminal 21 to the other end maycoincide with the extending direction of a short stub 31 b from theterminal 24 to a ground. On a multilayer substrate 50D shown in FIG. 8B,an open stub 30 d is disposed so that the extending direction of theopen stub 30 d from one end connected to the common terminal 21 to theother end may coincide with the extending direction of the short stub 31b from a ground to the terminal 24.

FIG. 9 is a graph illustrating the simulation results of isolationcharacteristics between the filter circuits in the configurations shownin FIGS. 8A and 8B. FIG. 9 shows that almost no differences are observedin the isolation characteristics, regardless of the direction in whichthe open stubs 30 c and 30 d overlap the short stub 31 b. It isvalidated from FIG. 9 that an improvement in the isolationcharacteristics is not influenced by the overlapping directions of theopen stubs 30 c and 30 d on the short stub 31 b.

FIG. 10 is a conceptual diagram illustrating the configuration of aduplexer 100C according to a third embodiment of the disclosure. Theduplexer 100C is different from the duplexer 100A shown in FIG. 1 inthat it also includes a matching network 60. Details of theconfigurations of the filter circuits 10 and 11 and the stubs 30 and 31are similar to those of the duplexer 100A, and a detailed explanationthereof will thus be omitted.

The matching network 60 is connected between the common terminal 21 andthe antenna terminal 20. The matching network 60 performs impedancematching between the output impedance of each of the filter circuits 10and 11 and the input impedance of an antenna (not shown).

The stub connected to the common terminal 21 may be used for performingimpedance matching between a duplexer and an antenna (not shown), aswell as for improving the isolation characteristics. It is now assumedthat a short stub is connected to the common terminal 21, as in theconfiguration of the related art. In this case, to improve the isolationcharacteristics and also to perform impedance matching, the line-lengthof the short stub is required to be increased to raise the inductancevalue of the short stub. In contrast, an open stub is less likely toinfluence the impedance characteristics in the frequency band to be usedthan a short stub. In the duplexer 100C, even if the line-length of theopen stub 30 is decreased, the open stub 30 less influences the matchingnetwork 60 than in the configuration of the related art. In this manner,it is possible to separately control impedance matching of the matchingnetwork 60 and improving of the isolation characteristics in theduplexer 100C.

A slight change may be made to the impedance characteristics by theadjustment of the line-length or the line-width of the open stub 30. Inother words, in the duplexer 100C, the open stub 30 may be used foradjusting impedance matching.

Instead of using both of the matching network 60 and the open stub 30 toperform impedance matching as shown in FIG. 10, the open stub 30 may beused to perform impedance matching in place of the matching network 60.

FIG. 11 is a Smith chart illustrating a path of output impedance of thefilter circuit 10 in the duplexer 100C. More specifically, FIG. 11illustrates a path of output impedance of the filter circuit 10 in theconfiguration with the open stub 30 and that in the configurationwithout the open stub 30. The frequency of a transmit signal is variedfrom 880 to 915 MHz, which is the pass band of the filter circuit 10.

FIG. 11 shows that, although the output impedance of the filter circuit10 has frequency characteristics, the path of the output impedance inthe configuration with the open stub 30 and that without the open stub30 almost the same in the entire frequency range. It is thus validatedthat using the open stub 30 in the duplexer 100C does not significantlyinfluence the impedance characteristics in the frequency band to beused.

FIG. 12 is a conceptual diagram illustrating the configuration of amultiplexer 100D according to a fourth embodiment of the disclosure. Themultiplexer 100D includes three duplexers 100A shown in FIG. 1 and aswitch circuit 70 for switching electrical connection between theantenna terminal 20 and one of the duplexers 100A.

More specifically, the multiplexer 100D includes duplexers 100Ax, 100Ay,and 100Az which support three different frequency bands. Theconfiguration of the duplexers 100Ax, 100Ay, and 100Az is similar tothat of the duplexer 100A shown in FIG. 1, and the elements of theduplexers 100Ax, 100Ay, and 100Az are designated by like referencenumerals and an explanation thereof will thus be omitted. In themultiplexer 100D, one of the three duplexers 100Ax, 100Ay, and 100Az isoperated in accordance with the frequency band of a transmit signal andthat of a received signal. Although the multiplexer 100D includes thethree duplexers 100Ax, 100Ay, and 100Az in FIG. 12, it may include moreor fewer separators 100A.

The switch circuit 70 is a three-input-and-one-output switch. The switchcircuit 70 outputs a transmit signal supplied from one of the filtercircuits 10 x, 10 y, and 10 z to the matching network 60, and outputs areceived signal supplied from the matching network 60 to one of thefilter circuits 11 x, 11 y, and 11 z. The switch circuit 70 may be ann-input-and-n-output switch (n is an integer).

If the stubs 30 x, 30 y, and 30 z are constituted by short stubs, theimpedance characteristics are likely to change, as discussed above. Itis thus necessary to adjust the constant of the matching network 60according to the inductance value of the short stub. Accordingly, asmany matching networks as filter circuits 10 are required, that is, theduplexer includes plural matching networks. In contrast, in the fourthembodiment, the stubs 30 x, 30 y, and 30 z are constituted by openstubs, which are less likely to influence the impedance characteristics,as shown in FIG. 11. It is thus possible to optimize the inductancevalue of the stub according to the frequency band to be used withoutnecessarily significantly influencing the impedance matching performedby the matching network 60. With this configuration, in the multiplexer100D supporting multiple frequency bands, only one matching network 60is required at a stage following the switch circuit 70. It is thuspossible to reduce the circuit scale of the multiplexer 100D to besmaller than the configuration in which the stubs 30 x, 30 y, and 30 zare constituted by short stubs.

The switch circuit 70 in FIG. 12 connects one of three inputs to theantenna terminal 20. The switch circuit 70 may alternatively connectsome or all of three inputs to the antenna terminal 20 at the same time.In this case, the multiplexer 100D supports carrier aggregation in whichthe plural duplexers 100Ax, 100Ay, and 100Az perform communication atthe same time. In carrier aggregation, it is desirable to improve theisolation characteristics among filter circuits in different frequencybands, as well as to improve the isolation characteristics betweentransmit and receive filter circuits (filter circuits 10 x and 11 x, forexample) in the same frequency band. An embodiment of the disclosure isthus more effectively applied to a duplexer supporting carrieraggregation.

In FIG. 12, the open stubs 30 x, 30 y, and 30 z are respectivelydisposed between the common terminals 21 x, 21 y, and 21 z and theswitch circuit 70. However, the open stubs 30 x, 30 y, and 30 z may bedisposed between the switch circuit 70 and the antenna terminal 20.

Various modified examples of duplexers according to embodiments of thedisclosure will be described below with reference to FIGS. 13A through17B.

FIGS. 13A through 13C are sectional views illustrating modified examplesregarding the arrangement of the open stub 30 and the short stub 31. Themultilayer substrates shown in FIGS. 13A through 13C are viewed from adirection similar to that of FIG. 4.

In a multilayer substrate 50E in FIG. 13A, part of an open stub 30 e(X-axis positive direction in FIG. 13A) formed in the front layer 53 andpart of a short stub 31 c (X-axis negative direction in FIG. 13A) formedin the inner layer 52 overlap each other in the Z-axis direction. In amultilayer substrate 50F in FIG. 13B, an open stub 30 f formed in thefront layer 53 and a short stub 31 d formed in the inner layer 52 do notoverlap each other in the Z-axis direction but are displaced from eachother. In a multilayer substrate 50G in FIG. 13C, an open stub 30 g anda short stub 31 e are both formed in the inner surface 52 and aredisposed adjacent to each other.

In this manner, the arrangement of the open stub 30 and the short stub31 is not restricted to a particular arrangement, provided that the openstub 30 and the short stub 31 are magnetically coupled with each other.For example, the open stub 30 and the short stub 31 may overlap eachother without necessarily being displaced in a plan view of the mainsurface of the multilayer substrate, as in the multilayer substrate 50Ain FIG. 4. Alternatively, the open stub 30 and the short stub 31 maypartially be displaced from each other, as shown in FIG. 13A, or mayentirely be displaced from each other, as shown in FIG. 13B. The openstub 30 and the short stub 31 may alternatively be disposed in the samelayer side by side, as shown in FIG. 13C.

The layers in or on which the open stub 30 and the short stub 31 areformed are not limited to particular layers. The positions of the openstub 30 and the short stub 31 shown in FIGS. 13A through 13C may bereplaced by each other. The position of the short stub 32 shown in FIG.5 may be arranged similarly to the short stub 31, and a detailedexplanation thereof will be omitted.

FIG. 14 is a plan view illustrating a modified example regarding thearrangement of the open stub 30 and the short stub 31. The multilayersubstrate in FIG. 14 is viewed from a direction similar to that of FIG.3.

On a multilayer substrate 50H in FIG. 14, an open stub 30 h and theshort stub 31 b are formed on the same layer, such as those shown inFIG. 13C. More specifically, the short stub 31 b is formed to extendcounterclockwise in a spiral shape, while the open stub 30 h is formedto extend counterclockwise to follow the three sides of the short stub31 b. The open stub 30 h and the short stub 31 b are formed side by sidesubstantially in parallel with each other in a region where they areadjacent to each other. Magnetic coupling between the open stub 30 andthe short stub 31 is achieved when they are placed in close proximity toeach other. In this case, the open stub 30 and the short stub 31 may notnecessarily be adjacent to each other on different layers but may bedisposed on the same layer if they extend side by side substantially inparallel with each other.

FIGS. 15A and 15B are plan views illustrating modified examplesregarding the arrangement of the open stub 30 and the short stub 31.FIG. 16 is a graph illustrating the simulation results of isolationcharacteristics between the filter circuits in the configurations shownin FIGS. 15A and 15B. The open stub 30 and the short stub 31 on amultilayer substrate 50I in FIG. 15A overlap each other in a differentmanner from those on a multilayer substrate 50J in FIG. 15B.

More specifically, an open stub 30 j formed on the multilayer substrate50J overlaps the short stub 31 b from a portion close to the terminal24, unlike an open stub 30 i formed on the multilayer substrate 50I.That is, the overlapping amount between the open stub 30 and the shortstub 31 in FIG. 15B is smaller than that in FIG. 15A. FIG. 16 shows thatthe isolation characteristics are improved to be higher when the openstub 30 overlaps the short stub 31 from the end which is not grounded(that is, from the terminal 24). The line-length of the open stub 30 jon the multilayer substrate 50J can thus be made shorter than that ofthe open stub 30 i on the multilayer substrate 50I.

FIGS. 17A and 17B are plan views illustrating modified examplesregarding the line-width of the short stub 31. The multilayer substratesin FIGS. 17A and 17B are viewed from a direction similar to that of FIG.3. The open stub 30 is not shown in FIGS. 17A and 17B for the sake ofrepresentation.

The line-width of a short stub 31 g formed on a multilayer substrate 50Lin FIG. 17B is narrower than that of a short stub 31 f formed on amultilayer substrate 50K in FIG. 17A. Decreasing the line-width of theshort stub 31 can increase the number of turns of spiral winding ordecrease the distance between opposing portions of spiral winding. Thiscan raise the self-inductance value of the short stub 31. Adjusting ofthe self-inductance values of the short stubs 31 f and 31 g can changethe coupling strength with the open stub 30. The line-width of the shortstub 31 may be narrower than that of the open stub 30, for example.

In all the above-described duplexers 100A through 100C and multiplexer100D, the short stub 31 (and the short stub 32 in the duplexer 100B) isconnected to the filter circuit 10 on the transmit side to reduce aleakage of a transmit signal to the filter circuit 11 on the receiveside. In a duplexer, however, a received signal may also leak to thetransmit filter circuit 10. The configurations (fifth through seventhembodiments) in which a leakage of a received signal to the filtercircuit 10 can be reduced will be discussed below with reference toFIGS. 18 through 23.

FIG. 18 is a conceptual diagram illustrating the configuration of aduplexer 200A according to a fifth embodiment of the disclosure. Theduplexer 200A is different from the duplexer 100A in FIG. 1 in that itincludes a short stub 33 instead of the short stub 31.

The short stub 33 (second wiring) is connected at one end to a resonatorincluded in the filter circuit 11, which will be discussed later, and isgrounded at the other end. The configuration of the open stub 30 (firstwiring) is similar to that of the open stub 30 shown in FIG. 1. The openstub 30 and the short stub 33 are magnetically coupled with each other.

The specific configuration of the filter circuit 11 (first filtercircuit) on the receive side is different from that of the filtercircuit 10 (second filter circuit) on the transmit side shown in FIG. 2.This will be discussed below with reference to FIG. 19.

FIG. 19 is a circuit diagram illustrating the configuration of a filtercircuit 11A and that of the stubs 30 and 33 in the duplexer 200A of thefifth embodiment.

The filter circuit 11A includes plural filter elements. Morespecifically, the filter circuit 11A includes resonators 80 through 84,85, and 86. The five resonators 80 through 84 are vertically connectedbetween the receive terminal 23 and the common terminal 21. Theresonator 85 is connected in series with a line L2 which connects thereceive terminal 23 and the common terminal 21. The resonator 86 isconnected between the line L2 and a ground. The configuration of theresonators 80 through 86 is not limited to a particular configuration.As in the resonators 41 through 47, for example, the resonators 80through 86 may be SAW filters, filters, such as piezoelectric thin-filmresonators, BAW filters, or I.H.P. SAW filters. Five resonators 80through 84, one resonator 85, and one resonator 86 are provided in FIG.19. However, more or fewer resonators may be provided.

One end of each of the vertically connected resonators 80 through 84 isconnected to the common terminal 21 via the resonator 85 or to thereceive terminal 23. More specifically, the ends of the resonators 80through 84 are alternately connected to the common terminal 21 and tothe receive terminal 23. The other ends of the resonators 80 through 84are grounded. The resonator 86 (first resonator) is connected at one endto the line L2 so as to branch off from the line L2 and at the other end(terminal 26) to one end of the short stub 33.

The short stub 33 is connected at one end to the terminal 26 and isgrounded at the other end. The short stub 33 is formed together with theopen stub 30 on the module substrate 50 and is magnetically coupled withthe open stub 30. The arrangement of the open stub 30 and the short stub33 on the module substrate 50 may be similar to that of the open stub 30a and the short stub 31 a on the multilayer substrate 50A shown in FIG.3, for example, and a detailed explanation thereof will thus be omitted.

In a manner similar to the duplexers 100A through 100C and multiplexer100D, the duplexer 200A configured as described above is able toincrease the attenuation of signals that are not included in the passband of the filter circuit 11A, compared with the configuration withoutthe stubs 30 and 33. The duplexer 200A is thus able to improve theisolation characteristics between the filter circuits 10 and 11.

FIG. 20 is a circuit diagram illustrating the configuration of a filtercircuit 11B and that of stubs 30, 33, and 34 in a duplexer 200Baccording to a sixth embodiment. The duplexer 200B is different from theduplexer 200A shown in FIG. 19 in that it also includes a short stub 34.

One end of the short stub 34 (third wiring) is connected to a terminal27 on the side on which the resonators 80 through 84 (second resonators)are grounded. The other end of the short stub 34 is grounded. That is,the short stub 34 is disposed between the resonators 80 through 84 and aground. The short stub 34, as well as the short stub 33, is formedtogether with the open stub 30 on the module substrate 50 and ismagnetically coupled with the open stub 30.

FIG. 21 is a plan view of a main surface of a multilayer substrate 50Mon which the duplexer 200B of the sixth embodiment is formed.

On the multilayer substrate 50M shown in FIG. 21, short stubs 33 a and34 a (corresponding to the short stubs 33 and 34 in FIG. 20) are formedon a different layer from that of an open stub 30 k (corresponding tothe open stub 30 in FIG. 20). More specifically, the short stub 33 a isconnected at one end to the terminal 26 and extends in the Y-axisnegative direction in a plan view of the main surface of the multilayersubstrate 50M. The short stub 34 a is connected at one end to theterminal 27 and extends in the X-axis positive direction in a plan viewof the main surface of the multilayer substrate 50M. The short stubs 33a and 34 a contact each other on the multilayer substrate 50M. The otherends of the short stubs 33 a and 34 a are both connected to a ground viavia-holes.

The open stub 30 k is connected at one end to the common terminal 21 andextends to overlap at least part of each of the short stubs 33 a and 34a in the Z-axis direction. More specifically, the open stub 30 k has abranch point at a mid-portion and branches off into two portions towardthe other end, which is the open end, from this branch point. These twoportions overlap the short stubs 33 a and 34 a. With this configuration,the open stub 30 k is magnetically coupled with each of the short stubs33 a and 34 a.

The duplexer 200B configured as described above also achieves advantagessimilar to those of the duplexer 200A. The duplexer 200B also includesthe short stub 34 so that it can further attenuate signals that are notincluded in the pass band of the filter circuit 11B. The duplexer 200Bis thus able to further improve the isolation characteristics than theduplexer 200A.

Although the open stub 30 k has a branch point at a mid-portion in FIG.21, it may branch off into two portions from the common terminal 21.

The arrangement of the open stub 30 and the short stubs 33 and 34 is notrestricted to a particular arrangement and may be similar to that of theopen stub 30 b and the short stubs 31 a and 32 a on the multilayersubstrate 50B shown in FIG. 6, for example.

Although the duplexer 200B includes the two short stubs 33 and 34 inFIG. 20, it may include only one of the short stubs 33 and 34.

FIG. 22 is a circuit diagram illustrating the configuration of a filtercircuit 11C and that of stubs 30, 33, 34, and 35 in a duplexer 200Caccording to a seventh embodiment. The duplexer 200C is different fromthe duplexer 200B shown in FIG. 20 in that it also includes a stub 35.

One end of the short stub 35 (third wiring) is connected to a terminal28 on the side on which the resonators 80 through 84 (second resonators)are grounded. The other end of the short stub 35 is grounded. That is,the short stub 35 is disposed between the resonators 80 through 84 and aground. The short stub 35, as well as the short stub 34, is formedtogether with the open stub 30 on the module substrate 50 and ismagnetically coupled with the open stub 30.

FIG. 23 is a plan view of a main surface of a multilayer substrate 50Non which the duplexer 200C of the seventh embodiment is formed.

On the multilayer substrate 50N shown in FIG. 23, a short stub 35 a(corresponding to the short stub 35 in FIG. 22) is formed on a differentlayer from that of an open stub 30 l (corresponding to the open stub 30in FIG. 22). More specifically, the short stub 35 a is connected at oneend to the terminal 28 and extends in the Y-axis negative direction in aplan view of the main surface of the multilayer substrate 50N. The shortstub 35 a then contacts the short stubs 33 a and 34 a on the multilayersubstrate 50N and is connected at the other end to a ground via avia-hole.

The open stub 30 l is connected at one end to the common terminal 21 andextends to overlap at least part of each of the short stubs 33 a, 34 a,and 35 a in the Z-axis direction. More specifically, the open stub 30 lhas a branch point at a mid-portion and branches off into three portionstoward the other end, which is the open end, from this branch point.These three portions overlap the short stubs 33 a, 34 a, and 35 a. Withthis configuration, the open stub 30 l is magnetically coupled with eachof the short stubs 33 a, 34 a, and 35 a.

The duplexer 200C configured as described above also achieves advantagessimilar to those of the duplexers 200A and 200B. The duplexer 200C alsoincludes the short stub 35 so that it can further attenuate signals thatare not included in the pass band of the filter circuit 11B. Theduplexer 200C is thus able to further improve the isolationcharacteristics than the duplexer 200B.

Although the open stub 30 l has a branch point at a mid-portion in FIG.23, it may branch off into three portions from the common terminal 21.

Although the duplexer 200C includes the three short stubs 33, 34, and 35in FIG. 22, it may include only one or two of the short stubs 33, 34,and 35.

FIG. 24 is a graph illustrating the simulation results of isolationcharacteristics between the filter circuits in the duplexer 200A. Morespecifically, FIG. 24 shows the results of comparing the isolationcharacteristics between the filter circuits 10 and 11 in theconfiguration of the duplexer 200A with the open stub 30 and those ofthe configuration without the open stub 30. In this graph, the verticalaxis indicates the isolation characteristics (dB), while the horizontalaxis indicates the frequency (GHz). The above-described simulations havebeen conducted, assuming that the duplexer 200A is used for transmittingand receiving Band-11 RF signals. The pass band of the filter circuit 10is 1427.9 to 1447.9 MHz, while the pass band of the filter circuit 11 is1475.9 to 1495.9 MHz.

FIG. 24 shows that the isolation characteristics are improved in aregion which is about half the transmit frequency band (1427.9 to 1447.9MHz) in the configuration with the open stub 30. In the receivefrequency band (1475.9 to 1495.9 MHz), almost no differences areobserved in the isolation characteristics between the configuration withthe open stub 30 and that without the open stub 30. That is, it ispossible to improve the isolation characteristics in the transmitfrequency band by the insertion of the open stub 30 substantiallywithout necessarily influencing the receive frequency band.

FIG. 25 is a graph illustrating the simulation results of isolationcharacteristics between the filter circuits in the duplexer 200C. Morespecifically, FIG. 25 shows the results of comparing the isolationcharacteristics between the filter circuits 10 and 11 in theconfiguration of the duplexer 200C with the open stub 30 and those ofthe configuration without the open stub 30. In this graph, the verticalaxis indicates the isolation characteristics (dB), while the horizontalaxis indicates the frequency (GHz). The pass bands of the filtercircuits 10 and 11 are similar to those of the duplexer 200A in FIG. 24.

FIG. 25 shows that, in the configuration with the open stub 30, theisolation characteristics are significantly improved at and around aspecific frequency (about 1444 MHz) within the transmit frequency band(1427.9 to 1447.9 MHz), though the isolation characteristics aredecreased in a certain frequency range. In the receive frequency band(1475.9 to 1495.9 MHz), almost no differences are observed in theisolation characteristics between the configuration with the open stub30 and that without the open stub 30. That is, it is possible to improvethe isolation characteristics in the transmit frequency band by theinsertion of the open stub 30 substantially without necessarilyinfluencing the receive frequency band. The duplexer 200C includes theplural short stubs 33 through 35. Hence, by suitably adjusting theinductance values of the short stubs 33 through 35 and the couplingstrength thereof with the open stub 30, a signal of a desired frequencycan be attenuated.

FIG. 26 is a conceptual diagram illustrating the configuration of aduplexer 200D according to a modified example of the fifth embodiment.FIG. 27 is a conceptual diagram illustrating the configuration of aduplexer 200E according to another modified example of the fifthembodiment. Both of the duplexers 200D and 200E shown in FIGS. 26 and 27have a configuration in which the duplexers 100A and 200A are combinedwith each other.

More specifically, in the duplexer 200D, the short stub 31 connected tothe filter circuit 10 and the short stub 33 connected to the filtercircuit 11 are both magnetically coupled with the single open stub 30.In contrast, the duplexer 200E includes two open stubs 30A and 30B. Theshort stub 31 connected to the filter circuit 10 is magnetically coupledwith the open stub 30A, while the short stub 33 connected to the filtercircuit 11 is magnetically coupled with the open stub 30B. The specificconfiguration in which the open stub 30 and the short stubs 31 and 33are connected to each other is similar to that of the above-describedembodiments, and an explanation thereof will thus be omitted.

The duplexers 200D and 200E configured as described above can reduce asignal leaking from the filter circuit 10 to the filter circuit 11 and asignal leaking from the filter circuit 11 to the filter circuit 10,compared with the configuration without the stub 30 (stubs 30A and 30B),31, and 33. The duplexers 200D and 200E are thus able to improve theisolation characteristics between the filter circuits 10 and 11.

FIGS. 26 and 27 show that there are two combinations of the open stub 30and the short stubs 31 and 33 in both of the duplexers 200D and 200E.However, there may be three or more combinations of the open stub andshort stubs in the duplexers.

In all of the above-described duplexers 100A through 100D and 200Athrough 200E, the use of filter circuits that separate a transmit signaland a received signal from each other according to the frequency isassumed. That is, the application of frequency division duplexing (FDD)using different frequency bands to transmit and receive signals to acellular phone is assumed. However, time division duplexing (TDD) mayalso be applicable to a cellular phone in which the same frequency bandis used to transmit and receive signals in different times. Adescription will now be given, with reference to FIG. 28, of theconfiguration (eighth embodiment) that is capable of reducing a leakageof signals between filter circuits with the application of both of FDDand TDD.

FIG. 28 is a conceptual diagram illustrating the configuration of afront-end circuit 300A according to an eighth embodiment of thedisclosure. The front-end circuit 300A includes an FDD circuit 310, aTDD circuit 320, a switch circuit 71, an open stub 36, and a short stub37.

The FDD circuit 310 includes the configuration of the duplexer 100Ashown in FIG. 1. The filter circuit 10 (first filter circuit) allows atransmit signal Tx1 (first transmit signal) of a predetermined frequencyband to pass therethrough from the transmit terminal 22 (first terminal)to the common terminal 21. The filter circuit 11 (second filter circuit)allows a received signal Rx1 (first received signal) of this frequencyband to pass therethrough from the common terminal 21 to the receiveterminal 23 (second terminal). In the eighth embodiment, the commonterminal 21 is connected to one output terminal of the switch circuit71.

The TDD circuit 320 includes a filter circuit 12 and terminals 90 and91. The filter circuit 12 (third filter circuit) has frequencycharacteristics that allow a transmit signal Tx2 (second transmitsignal) and a received signal Rx2 (second received signal) of apredetermined frequency band to pass through the filter circuit 12between the terminal 90 (third terminal) on the transmit side and theterminal 91 (fourth terminal) on the antenna side. One of the transmitsignal Tx2 and the received signal Rx2 is supplied to the filter circuit12 according to the time. The terminal 91 is connected to the otheroutput terminal of the switch circuit 71.

The filter circuit 12 includes one or plural resonators, as in theabove-described filter circuits 10A, 10B, and 11A through 11C. Thespecific configuration of the filter circuit 12 is similar to that ofone of the above-described filter circuits, and a detailed explanationthereof will thus be omitted.

The switch circuit 71 is a one-input-and-two-output switch. The inputterminal of the switch circuit 71 is connected to the antenna terminal20, while one of the output terminals is connected to the FDD circuit310 and the other output terminal is connected to the TDD circuit 320.The switch circuit 71 connects one of the FDD circuit 310 and the TDDcircuit 320 to the antenna terminal 20 or both of the FDD circuit 310and the TDD circuit 320 to the antenna terminal 20 at the same time, inaccordance with a control signal supplied from the outside of thefront-end circuit 300A. When the switch circuit 71 connects both of theFDD circuit 310 and the TDD circuit 320 to the antenna terminal 20, thefront-end circuit 300A supports carrier aggregation in which FDD and TDDcommunication can be performed at the same time.

The open stub 36 (first wiring) is connected at one end between theterminal 91 and the switch circuit 71 and is opened at the other end.

The short stub 37 (second wiring) is connected at one end to a resonator(not shown) included in the filter circuit 12 and is grounded at theother end. The open stub 36 and the short stub 37 are magneticallycoupled with each other.

In a manner similar to the duplexers 100A through 100D and 200A through200E, the front-end circuit 300A configured as described above can alsoincrease the attenuation of signals that are not included in the passband of the filter circuit 12 of the TDD circuit 320, compared with theconfiguration without the stubs 36 and 37. The front-end circuit 300A isthus able to improve the isolation characteristics between the FDDcircuit 310 and the TDD circuit 320.

Although the stubs 36 and 37 are provided on the side of the TDD circuit320 in FIG. 28, they may alternatively be provided on the side of thefilter circuits 10 and 11 of the FDD circuit 310. Stubs mayalternatively be provided for both of the FDD circuit 310 and the TDDcircuit 320, as in the configurations shown in FIGS. 26 and 27.

Although the front-end circuit 300A includes one FDD circuit 310 and oneTDD circuit 320 in FIG. 28, it may include two or more FDD circuits 310and two or more TDD circuits 320.

The embodiments of the disclosure have been discussed above. Theduplexers 100A through 100D each include filter circuits 10 and 11, anopen stub 30, and a short stub 31. The open stub 30 is connected at oneend to the common terminal 21 and is opened at the other end. The shortstub 31 is connected at one end to the resonator 45 of the filtercircuit 10 and is grounded at the other end. The open stub 30 and theshort stub 31 are magnetically coupled with each other. With thisconfiguration, signals that are not included in the pass band of thefilter circuit 10 flow to the stub 31, and the attenuation of suchsignals is increased, compared with the configuration without the stubs30 and 31. The duplexers 100A through 100D are thus able to improve theisolation characteristics between plural filter circuits whileincreasing the flexibility in the arrangement of wirings.

The duplexers 200A through 200E each include filter circuits 10 and 11,an open stub 30, and a short stub 33. The open stub 30 is connected atone end to the common terminal 21 and is opened at the other end. Theshort stub 33 is connected at one end to the resonator 86 of the filtercircuit 11 and is grounded at the other end. The open stub 30 and theshort stub 33 are magnetically coupled with each other. With thisconfiguration, signals that are not included in the pass band of thefilter circuit 11 flow to the stub 33, and the attenuation of suchsignals is increased, compared with the configuration without the stubs30 and 33. The duplexers 200A through 200E are thus able to improve theisolation characteristics between plural filter circuits whileincreasing the flexibility in the arrangement of wirings.

In the duplexers 100B and 200B, the filter circuits 10B and 11B alsoinclude the short stubs 32 and 34, respectively. The attenuation ofsignals that are not included in the pass bands of the filter circuits10B and 11B is further increased. The duplexers 100B and 200B are thusable to improve the isolation characteristics to be higher than theduplexers 100A and 200A.

In the duplexers 100A through 100D, the open stub 30 and the short stub31 may be formed in or on different layers of a multilayer substrate. Inthe duplexers 200A through 200E, the open stub 30 and the short stub 33may be formed in or on different layers of a multilayer substrate. Then,the open stub 30 and each of the short stubs 31 and 33 may be formed toat least partially overlap each other. This establishes magneticcoupling between the open stub 30 and each of the short stubs 31 and 33.

In the duplexers 100A through 100D, the open stub 30 and the short stub31 may be formed in or on the same layer of a multilayer substrate. Inthe duplexers 200A through 200E, the open stub 30 and the short stub 33may be formed in or on the same layer of a multilayer substrate. Then,the open stub 30 and each of the short stubs 31 and 33 may at leastpartially be disposed substantially in parallel with and adjacent toeach other. This establishes magnetic coupling between the open stub 30and each of the short stubs 31 and 33.

Although the stubs of the duplexers 100A through 100D and 200A through200E are not limited to a specific configuration, they may beconstituted by lumped elements.

The front-end circuit 300A includes filter circuits 10 through 12, anopen stub 36, and a short stub 37. The open stub 36 is connected at oneend to the terminal 91 and is opened at the other end. The short stub 37is connected at one end to a resonator of the filter circuit 12 and isgrounded at the other end. The open stub 36 and the short stub 37 aremagnetically coupled with each other. With this configuration, signalsthat are not included in the pass band of the filter circuit 12 flow tothe stub 37, and the attenuation of such signals is increased, comparedwith the configuration without the stubs 36 and 37. The front-endcircuit 300A is thus able to improve the isolation characteristicsbetween plural filter circuits while increasing the flexibility in thearrangement of wirings.

The above-described embodiments are provided for facilitating theunderstanding of the disclosure but are not intended to be exhaustive orto limit the disclosure to the precise forms disclosed. Modificationsand/or improvements may be made without departing from the scope andspirit of the disclosure, and equivalents of the disclosure are alsoencompassed in the disclosure. That is, suitable design changes made tothe embodiments by those skilled in the art are also encompassed in thedisclosure within the scope and spirit of the disclosure. For example,the elements of the embodiments and the positions, materials,conditions, configurations, and sizes thereof are not restricted tothose described in the embodiments and may be changed in an appropriatemanner. The elements of the embodiments may be combined within atechnically possible range, and configurations obtained by combining theelements of the embodiments are also encompassed in the disclosurewithin the scope and spirit of the disclosure.

For example, in the above-described embodiments, the stub connected tothe common terminal 21 or the terminal 91 is an open stub, while thestub connected to a resonator of a filter circuit is a short stub.However, this is only an example. The stub connected to the commonterminal 21 or the terminal 91 may be a short stub, while the stubconnected to a resonator of a filter circuit may be an open stub.

In the above-described embodiments, the open stub and the short stub aremagnetically coupled with each other by electromagnetic induction.However, the open stub and the short stub may be electromagneticallycoupled with each other by electric-field induction instead ofmagnetic-field induction. More specifically, by increasing the area of ametal layer forming the open stub and the short stub in a multilayersubstrate, the coupling capacitance may be formed. In this manner,electromagnetic coupling between the open stub and the short stub may beimplemented by magnetic-field induction, electric-field induction, or acombination thereof.

Some elements in the above-described embodiments may be combined witheach other. For example, the configuration in which the open stub has abranch point, as shown in FIGS. 21 and 23, may be applied to theduplexers 100A through 100C, multiplexer 100D and duplexer 200A, 200D,and 200E or the front-end circuit 300A.

While embodiments of the disclosure have been described above, it is tobe understood that variations and modifications will be apparent tothose skilled in the art without departing from the scope and spirit ofthe disclosure. The scope of the disclosure, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. A duplexer comprising: a first filter circuitconfigured to pass a signal of a first frequency band between a firstterminal and a common terminal, wherein the first filter circuitcomprises a first resonator and a line connecting the first terminal tothe common terminal, the first resonator being connected at a first endto the line so as to branch off from the line; a second filter circuitconfigured to pass a signal of a second frequency band between a secondterminal and the common terminal, the second frequency band beingdifferent from the first frequency band; a first wiring that isconnected at a first end to the common terminal and is open at a secondend; and a second wiring that is connected at a first end to a secondend of the first resonator and is grounded at a second end, wherein thefirst and second wirings are electromagnetically coupled with eachother.
 2. The duplexer according to claim 1, wherein the first filtercircuit is configured to pass the signal of the first frequency bandfrom the first terminal to the common terminal and the second filtercircuit is configured to pass the signal of the second frequency bandfrom the common terminal to the second terminal.
 3. The duplexeraccording to claim 1, wherein: the first filter circuit furthercomprises a second resonator, the second resonator being connected at afirst end to the line so as to branch off from the line, the duplexerfurther comprises a third wiring that is separate from the secondwiring, said third wiring being connected at a first end to a second endof the second resonator and is grounded at a second end, and the firstand third wirings are electromagnetically coupled with each other. 4.The duplexer according to claim 1, wherein: the first frequency band isa transmit frequency band within a predetermined frequency band; and thesecond frequency band is a receive frequency band within thepredetermined frequency band.
 5. The duplexer according to claim 1,wherein: the first frequency band is a receive frequency band within apredetermined frequency band; and the second frequency band is atransmit frequency band within the predetermined frequency band.
 6. Theduplexer according to claim 1, further comprising: a multilayersubstrate in or on which the first and second wirings are formed,wherein the first and second wirings are formed in or on differentlayers of the multilayer substrate and are disposed such that at leastpart of the first wiring and at least part of the second wiring overlapeach other as seen in a plan view of a main surface of the multilayersubstrate.
 7. The duplexer according to claim 1, further comprising: amultilayer substrate in or on which the first and second wirings areformed, wherein the first and second wirings are formed in or on a samelayer of the multilayer substrate and are disposed such that at leastpart of the first wiring and at least part of the second wiring aresubstantially parallel with and adjacent to each other.
 8. The duplexeraccording to claim 1, wherein the first wiring or the second wiring isconstituted by a lumped element.
 9. The duplexer according to claim 1,wherein the second end of the first wiring is not connected to the firstterminal or to the second terminal.
 10. A front-end circuit comprising:a first filter circuit configured to pass a first transmit signalbetween a first terminal and a common terminal; a second filter circuitconfigured to pass a first received signal between a second terminal andthe common terminal; a third filter circuit configured to pass a secondtransmit signal and a second received signal between a third terminaland a fourth terminal, wherein the third filter circuit comprises afirst resonator and a line connecting the third terminal to the fourthterminal, the first resonator being connected at a first end to the lineso as to branch off from the line; a switch circuit configured toconnect one of or both of the common terminal and the fourth terminal toan antenna terminal; a first wiring that is connected at a first end tothe fourth terminal and is open at a second end; and a second wiringthat is connected at a first end to a second end of the first resonatorand is grounded at a second end, wherein the first and second wiringsare electromagnetically coupled with each other.
 11. The front-endcircuit according to claim 10, wherein the second end of the firstwiring is not connected to the first terminal, to the second terminal,to the third terminal, to the fourth terminal, or to the antennaterminal.